1. Field of the Invention
The invention relates to a method of determining the playing time of a magnetic tape, on a supply reel and a take up reel in a magnetic-tape cassette.
2. Description of the Related Art
DE 28 56 509 A1, corresponding to U.S. Pat. No. 4,232,371, describes a method of indicating the length of the tape on one reel of a cassette comprising two reels. In this device, a loading mechanism extracts a specific length of tape from the cassette with a constant linear velocity, one of the two reels of the cassette being retained. A pulse generator then generates a pulse frequency proportional to the angular velocity. On the basis of the constant linear velocity, the tape thickness and the reel diameter, a microprocessor then calculates the available quantity of tape. In order to calculate the quantity of tape wound on the reel in a specific time T.sub.S, the following equation is used: ##EQU3## where v is the linear velocity, .delta. is the thickness of the magnetic tape, P.sub.S is the signal representing the angular velocity of said one reel, and R.sub.H is the diameter of the inner turn of the magnetic tape wound on this reel. A disadvantage of this known device is that one of the two reels has to be retained by a special braking device.
Moreover, DE 36 22 895 A1 describes a method of determining the residual tape length of a magnetic tape wound on a reel spindle, wherein a time measurement device first determines the revolution times t.sub.L, t.sub.R of one full revolution of the two tape spools, from which the total time t=t.sub.L +t.sub.R is derived. Subsequently, by comparing the total time t with a given limit value, the reel spindle diameter D.sub.O for the relevant cassette is determined in order to calculate the number of turns n.sub.L and n.sub.R in accordance with the equations: ##EQU4## where .upsilon. is the standard velocity and d is the thickness of the magnetic tape. From the numbers of turns thus obtained, the winding times T.sub.L and T.sub.R are then derived in accordance with the equations ##EQU5## The sum of the two times T.sub.L and T.sub.R represents the residual time T from which the actual thickness of the magnetic tape ##EQU6## is calculated by comparison with possible nominal times. The actual thickness is finally used for the calculation of the real winding times.
Moreover, it is known from U.S. Pat. No. 4,963,999 to calculate the residual playing time T1 in accordance with the equation ##EQU7## Here, t is the thickness, .upsilon. is the velocity and L is the total length of the magnetic tape, R.sub.o is the radius of the reel hub, R.sub.t is the radius of the take-up spool and R.sub.s is the radius of the supply spool in the cassette. The velocity of the magnetic tape is derived from the angular velocity of a capstan driving the magnetic tape.